Predominant contribution of direct laser acceleration to high-energy electron spectra in a low-density self-modulated laser wakefield accelerator

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Predominant contribution of direct laser acceleration to high-energy electron spectra in a low-density self-modulated laser wakefield accelerator

P. M. King, K. Miller, N. Lemos, J. L. Shaw, B. F. Kraus, M. Thibodeau, B. M. Hegelich, J. Hinojosa, P. Michel, C. Joshi, K. A. Marsh, W. Mori, A. Pak, A. G. R. Thomas, and F. Albert

Phys. Rev. Accel. Beams 24, 011302 – Published 12 January 2021

Abstract

The two-temperature relativistic electron spectrum from a low-density ( $3 \times 10^{17} {cm}^{- 3}$ ) self-modulated laser wakefield accelerator (SM-LWFA) is observed to transition between temperatures of $19 \pm 0.65$ and $46 \pm 2.45 MeV$ at an electron energy of about 100 MeV. When the electrons are dispersed orthogonally to the laser polarization, their spectrum above 60 MeV shows a forking structure characteristic of direct laser acceleration (DLA). Both the two-temperature distribution and the forking structure are reproduced in a quasi-3D osiris simulation of the interaction of the 1-ps, moderate-amplitude ( $a_{0} = 2.7$ ) laser pulse with the low-density plasma. Particle tracking shows that while the SM-LWFA mechanism dominates below 40 MeV, the highest-energy ( $> 60 MeV$ ) electrons gain most of their energy through DLA. By separating the simulated electric fields into modes, the DLA-dominated electrons are shown to lose significant energy to the longitudinal laser field from the tight focusing geometry, resulting in a more accurate measure of net DLA energy gain than previously possible.

Received 24 August 2020
Accepted 4 January 2021

DOI:https://doi.org/10.1103/PhysRevAccelBeams.24.011302

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Laser wakefield acceleration Particle acceleration in plasmas

Particle-in-cell methods

Accelerators & Beams

Authors & Affiliations

P. M. King ^1,2,*, K. Miller ³, N. Lemos¹, J. L. Shaw ⁴, B. F. Kraus ⁵, M. Thibodeau¹, B. M. Hegelich², J. Hinojosa⁶, P. Michel¹, C. Joshi⁷, K. A. Marsh⁷, W. Mori³, A. Pak¹, A. G. R. Thomas⁶, and F. Albert¹

¹NIF and Photon Sciences, Lawrence Livermore National Laboratory, Livermore, California 94550, USA
²Department of Physics, University of Texas at Austin, Austin, Texas 78712, USA
³Department of Physics, University of California Los Angeles, Los Angeles, California 90095, USA
⁴Laboratory for Laser Energetics, University of Rochester, Rochester, New York 14623, USA
⁵Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08544, USA
⁶Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, Michigan 48109-2099, USA
⁷Department of Electrical Engineering, University of California Los Angeles, Los Angeles, California 90095, USA

^*king100@llnl.gov

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Issue

Vol. 24, Iss. 1 — January 2021

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Images

Figure 1
(a) Experimental setup. The electron beam is dispersed by a 0.6 T magnet onto two BAS-MS image plates after passing through three wire fiducials. A frequency-doubled probe beam is co-timed with the main pulse and provides on-shot interferometry of the plasma channel with a magnification of 3. (b) The electron energy spectrum for two different shots using identical laser and plasma parameters dispersed perpendicular and parallel to the laser polarization in blue and red, respectively, along with the simulated electron spectrum in green. All three spectra are fit to single-temperature distributions below ( $T_{1}$ ) and above ( $T_{2}$ ) 100 MeV; the two regions are separated by a dashed black line. The experimental spectra exhibit shot-to-shot variations of the single-shot laser system. (c) An undispersed electron beam profile.
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Figure 2
The ratio of the measured electron beam divergence parallel ( $θ_{∥}$ ) and perpendicular ( $θ_{⊥}$ ) to the linear laser polarization. Ratios greater than 1 indicate an elongation along the laser polarization.
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Figure 3
All the measured hot (cold), $T_{1}$ ( $T_{2}$ ) temperatures for the 4 (10) mm gas jet nozzles used in the experiment following the same analysis described in the main text body.
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Figure 4
Measured electron energy spectra for a plasma with electron density $n_{e} = 3 \times 10^{17} {cm}^{- 3}$ dispersed (a) perpendicular and (b) parallel to the linear laser polarization direction. The contrast is adjusted and a line-out along the dashed line is plotted (solid line) to emphasize the forking feature in the dispersed electron profile. The dashed black line indicates the acceptance aperture of the magnet. Note that (a) and (b) were taken on two different shots with similar laser energies.
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Figure 5
Snapshot of the electron density profile after 4.64 mm of propagation (left to right) through the plasma; $z$ and $x$ are the longitudinal and transverse directions, respectively. Also shown are the $m = 0$ longitudinal electric field (SM-LWFA) overlaid in red and the $m = 1$ transverse electric field envelope (DLA) in green. The dashed green line shows the vacuum laser field envelope at the focus. The tracked electrons, with $x$ positions given by their radial distance (only half-space is shown), indicate where in space each acceleration mechanism is dominant. The charge density has been integrated in $θ$ .
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Figure 6
Simulated electron spectra dispersed (a) perpendicular and (b) parallel to the linear laser polarization direction. A line-out along the dashed line is plotted (solid line) to emphasize the “horns” of the dispersed beam. (c) The final energy gain due to different field components and mechanisms is shown for numerous tracked electrons (solid lines showing the mean within 20-MeV windows). All data is shown after 4.68 mm of propagation.
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